Analgesic doses of morphine and viminol R2 increase striatal dopamine turnover rate but this increase does not appear related to either an increase in the firing rate of dopaminergic neurons or to the stimulation of postsynaptic dopaminergic receptors. Morphine, viminol R2, azidomorphine increase cGMP in striatum and accumbens but not in cerebellum. The changes of cGMP in striatum and accumbens are blocked by naloxone. Thus activation of the cGMP system can represent the specific postsynpatic response following an interaction of the opiate receptor with its antagonist.